We analyze a [C? II ] 158 μ m map obtained with the L2 GREAT receiver on SOFIA of the reflection nebula illuminated by the early B star S 1 in the ρ Oph A cloud core. This data set has been complemented with maps of CO(3–2),~(13)CO(3–2), and C~(18)O(3–2), observed as a part of the James Clerk Maxwell Telescope (JCMT) Gould Belt Survey, with archival HCO~(+)(4–3) JCMT data, as well as with [O? I ] 63 and 145 μ m imaging with Herschel /PACS. The [C? II ] emission is completely dominated by the strong emission from the photon dominated region (PDR) in the nebula surrounding S 1 expanding into the dense Oph A molecular cloud west and south of S 1. The [C? II ] emission is significantly blueshifted relative to the CO spectra and also relative to the systemic velocity, particularly in the northwestern part of the nebula. The [C? II ] lines are broader toward the center of the S 1 nebula and narrower toward the PDR shell. The [C? II ] lines are strongly self-absorbed over an extended region in the S 1 PDR. Based on the strength of the [~(13)C? II ] F = 2–1 hyperfine component, [C? II ] is significantly optically thick over most of the nebula. CO and~(13)CO(3–2) spectra are strongly self-absorbed, while C~(18)O(3–2) is single peaked and centered in the middle of the self-absorption. We have used a simple two-layer LTE model to characterize the background and foreground cloud contributing to the [C? II ] emission. From this analysis we estimated the extinction due to the foreground cloud to be ~9.9 mag, which is slightly less than the reddening estimated toward S 1. Since some of the hot gas in the PDR is not traced by low- J CO emission, this result appears quite plausible. Using a plane parallel PDR model with the observed [O? I ](145)/[C? II ] brightness ratio and an estimated FUV intensity of 3100–5000 G_(0)suggests that the density of the [C? II ] emitting gas is ~3–4? ×?10~(3)cm~(?3).
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